Re: [PATCH 13/18] io_uring: add file set registration

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Feb 05, 2019 at 12:08:25PM -0700, Jens Axboe wrote:
> Proof is in the pudding, here's the main commit introducing io_uring
> and now wiring it up to the AF_UNIX garbage collection:
> 
> http://git.kernel.dk/cgit/linux-block/commit/?h=io_uring&id=158e6f42b67d0abe9ee84886b96ca8c4b3d3dfd5
> 
> How does that look?

In a word - wrong.  Some theory: garbage collector assumes that there is
a subset of file references such that
	* for all files with such references there's an associated unix_sock.
	* all such references are stored in SCM_RIGHTS datagrams that can be
found by the garbage collector (currently: for data-bearing AF_UNIX sockets -
queued SCM_RIGHTS datagrams, for listeners - SCM_RIGHTS datagrams sent via
yet-to-be-accepted connections).
	* there is an efficient way to count those references for given file
(->inflight of the corresponding unix_sock).
	* removal of those references would render the graph acyclic.
	* file can _NOT_ be subject to syscalls unless there are references
to it outside of that subset.

unix_inflight() moves a reference into the subset
unix_notinflight() moves a reference out of the subset
activity that might add such references ought to call wait_for_unix_gc() first
(basically, to stall the massive insertions when gc is running).

Note that unix_gc() does *NOT* work in terms of dropping file references -
the primary effect is locating the SCM_RIGHTS datagrams that can be disposed
of and taking them out.  It simply won't do anything to your file references,
no matter what.  Add a printk into your ->release() and try to register io_uring
descriptor into itself, then close it.  And observe ->release() not being
called for that object.  Ever.

PS: The algorithm used by unix_gc() is basically this -

	grab unix_gc_lock (giving exclusion with unix_inflight/unix_notinflight
			   and stabilizing ->inflight counters)

	Candidates = {}
	for all unix_sock u such that u->inflight > 0
		if file corresponding to u has no other references
			Candidates += u

	/* everything else already is reachable; due to unix_gc_lock these
	   can't die or get syscall-visible references under us */
	Might_Die = Candidates

	/* invariant to maintain: for u in Candidates u->inflight will be equal
	   to the number of references from SCM_RIGHTS datagrams *except*
	   those immediately reachable from elements of Might_Die */

	for all u in Candidates
		for each file reference v in SCM_RIGHTS datagrams
					immediately reachable from u
			if v in Candidates
				v->inflight--

	To_Scan = ()	// stuff reachable from those must live
	for all u in Might_Die
		if u->inflight > 0
			queue u into To_Scan

	while To_Scan is non-empty
		u = dequeue(To_Scan)
		Might_Die -= u
		for each file reference v in SCM_RIGHTS datagrams
					immediately reachable from u
			if v in Candidates
				v->inflight++	// maintain the invariant
				if v in Might_Die
					queue v into To_Scan

	/* at that point nothing in Might_Die is reachable from the outside */

	/* restore the original values of ->inflight */
	for all u in Might_Die
		for each file reference v in SCM_RIGHTS datagrams
					immediately reachable from u
			if v in Candidates
				v->inflight++

	hitlist = ()
	for all u in Might_Die
		for each SCM_RIGHTS datagram D immediately reachable from u
			if D contains references to something in Candidates
				move D to hitlist
	/* all those datagrams would've never become reachable */

	drop unix_gc_lock

	discard all datagrams in hitlist.

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